Man-made activities are causing the extermination of animal and plant species, and this happens so fast that evolution, nature’s built in mechanism to ensure the survival of species, cannot keep up.

Mammals, in particular, are vulnerable. Researchers have warned that sans improvement in conservation efforts, many mammal species will be wiped out over the next 50 years and nature would need 3 to 5 million years to recover this lost biodiversity.

Earth has so far witnessed five mass extinctions in the past 450 million years. Dramatic changes on the planet caused the extinction of plant and animal life but after each of these extinctions, evolution would slowly fill the gaps with new species.

Experts say that the sixth mass extinction is now happening, but unlike the previous extinction events, this is not caused by natural disasters, but rather caused by human activities.

What makes this sixth extinction more alarming is how fast paced it is. Matt Davis from Aarhus University and colleagues calculated that the extinctions move too rapidly for evolution to keep up.

In a new study published in the journal PNAS, the researchers found that if mammals diversify at their normal rates, it will take between 5 and 7 million years to restore the level of biodiversity before modern humans evolved.

Analysis also found that it would take three to five millions years to reach the current levels of biodiversity.

“We use a birth–death tree framework to show that even if extinction rates slow to preanthropogenic background levels, recovery of phylogenetic diversity (PD) will likely take millions of years,” the researchers reported in their study, which was published on Oct. 15.

“These findings emphasize the severity of the potential sixth mass extinction and the need to avoid the loss of unique evolutionary history now.”

Davis and colleagues said that the significance of different species is not the same. Some animals such as the now extinct Thylacoleo and Macrauchenia were evolutionary distinct lineages with only a few close relatives. When they go extinct, they take whole branches of the evolutionary tree with them.

This means that their loss is not only the loss of the species but also the loss of unique ecological functions and millions of years of evolution that they represent.

“There are hundreds of species of shrew, so they can weather a few extinctions. There were only four species of sabre-toothed tiger; they all went extinct,” Davis said in a statement published by Eurekalert.